The invention relates to a movable core transducer, and more particularly to a transducer of the differential transformer type.
Transducers of the above type are used in various applications where the position and/or displacement of an object in translation is to be converted into an electric signal.
Transducers of the differential transformer type are disclosed for example in the U.S. Pat. Nos. 3,054,976 (Lipshutz) and 3,235,790 (Collins). Such transducers include a primary winding and two secondary windings wound around a passage in wich a core is movable, said core being connected to a member of which the position or displacement is to be measured. The secondary windings are connected to provide a differential output signal varying as a function of the position of the movable core. These known transducers show several drawbacks. Firstly, the relation between the output signal and the position or displacement to be measured is dependent on variation in temperature and in amplitude of the voltage applied to the primary winding. In addition, there is no means to check the integrity of the transducer from the value of the output signal; for example, a zero output signal can mean that the core is in a particular position but can also result from a defect in the windings.
In order to eliminate the above drawbacks, it has been suggested in the U.S. Pat. No. 4,339,739, to provide a transducer of the differential transformer type which can be "self-monitored" by forming each secondary winding in two parts: one secondary winding including a basic part and an additional part connected in phase and the other secondary winding including similar basic part and additional part but connected in antiphase (opposition of phase). The output signals v1, v2 at the separate output terminals of the two secondary windings are combined to deliver a differential output signal representative of the ratio r=(v1+v2)/(v1-v2). By designing the secondary windings such that the above ratio cannot reach the values +1 or -1, a self-monitored transducer can be obtained, because a value of r equal to +1 or -1 means that a cut-off has occurred in a secondary winding. However, the secondary windings are formed "two wires in hand", i.e. the corresponding basic parts and additional parts of the secondary windings are formed simultaneously by winding their two respective constituent wires simultaneously and side by side. As a result, the secondary windings are capacitively coupled and, in practice, the above ratio r has a value different from +1 or -1 even when one secondary winding is cut-off. The transducer cannot then be reliably self-monitored, all the more reason that the frequency of the current fed to the primary winding is high.